Not to say Barcelona have no chance. I know Barcelona's overall performance is not very good. Their performance is erratic. However, this team cannot be underestimated. They have several experienced players in their squad.
In the Champions League we have seen in the past that the favorite teams do not always win. A few years ago we saw a mediocre team like Chelsea win the Champions League title. Almost every team that qualified for the Round of 16 performed well in the group stage. So I would say there is absolutely no chance of underestimating the Barcelona team. I think Barcelona will win against Napoli.
Barcelona's condition, which is not at its best, raises a little doubt that this team can survive longer in the Champions League. Barcelona may be able to beat Napoli who are slumping in Serie A, but will struggle when facing tougher opponents in the next round. I believe anything can happen in football, if you look at the performance of other teams such as Real Madrid, Bayern Munich and Manchester City it will be difficult for Barcelona to get rid of these teams.
In the 3 teams you mentioned here, Barcelona does not even stand a draw talk more of securing a win against any of the clubs. I don't know what may be the cause of these poor performance, everyone knew Barcelona as one big club when it comes to the champions league matches but now with their very dull performance they won't secure a better spot on the list. Barcelona winning Napoli should not be something one could bet a straight win, since both teams are not doing well anything can happen.